1. Field of the Invention
The present invention relates to a selective plating apparatus and a selective plating method, and more particularly, a selective plating apparatus for applying an electrolytic plating to a metal member onto which a mask member is attached so as to expose predetermined parts of a surface of the metal member, and so as to cover the predetermined parts of the surface with a plated metal layer made of a desired metal, and a selective plating method using this selective plating apparatus.
2. Description of the Related Art
In a lead frame used in a semiconductor device as shown in FIG. 8, a plurality of leads 10a are formed on a belt-shaped metal member 11 so as to surround a stage 10 on which a semiconductor element is mounted.
In the lead frame shown in FIG. 8, respective electrode terminals of the semiconductor element mounted on the stage 10 and corresponding tip end portions of the leads 10a on the stage 10 side (sometimes referred simply to as “tip end portions of the leads 10a” hereinafter) are bonded via wire and connected electrically. In order to make sure of the electrical connection, normally selective electrolytic plating such as electrolytic silver plating is applied to a shaded portion shown in FIG. 8, i.e., a whole surface of the stage 10 and the tip end portions of the respective leads 10a. 
In JP-A-58-174589 (FIG. 3), for example, it has been proposed that the selective plating apparatus shown in FIG. 9 should be employed in such selective electrolytic plating.
In the selective plating apparatus shown in FIG. 9, the lead frame is put between a pressing plate 100 and a mask member 102. An overall surface of the stage 10 of the lead frame and the leads 10a are exposed to a space 110 formed when the mask member 102 is attached onto the lead frame.
In such mask member 102, a mask plate 102b in which a supply opening 104 and discharge openings 106 for the electrolysis solution are formed is sandwiched between mask plate holding members 102a, 102c, and thus a recess portion 108 is formed by the mask plate holding member 102a and the mask plate 102b. This recess portion 108 constitutes the space 110 when the lead frame is held between the pressing plate 100 and the mask member 102.
Also, an injection nozzle 112 for injecting electrolytic plating solution is provided immediately under the supply opening 104 which is formed in the mask plate 102b, at a right angle to the mask plate 102b. 
In addition, a mesh-like anode 114 connected to an anode of a DC power supply is held between the mask plate 102b and the mask plate holding member 102c. 
In this case, the lead frame is connected to a cathode of the DC power supply.
According to the selective plating apparatus shown in FIG. 9, the electrolytic plating solution injected from the injection nozzle 112 is passed through the mesh-like anode 114, then enters into the space 110 toward a center portion of the stage 10 of the lead frame from the supply opening 104 in the mask plate 102b. Then, the electrolytic plating solution moves toward the discharge openings 106 while depositing a plated metal on the overall surface of the stage 10 of the exposed lead frame and surfaces of the respective tip end portions of the leads 10a, and is discharged from the discharge openings 106.
However, according to the selective plating apparatus shown in FIG. 9, the plated metal layer that is thicker than the plated metal layers on the tip end portions of the leads 10a is formed on the stage 10, as set forth in JP-A-58-174589, page 2, left lower column, lines 9 to 12.
In this manner, according to the selective plating apparatus shown in FIG. 9, there is unevenness in thickness between the plated metal layer formed on the stage 10 and the plated metal layers formed on respective tip end portions of the leads 10a. 
As a consequence, the selective plating apparatus shown in FIG. 9 cannot be applied to the selective plating of which a uniform thickness of the plated metal layers formed on the stage 10 and respective tip end portions of the leads 10a is required.
Also, in some cases the electrolytic plating should be applied to a shaded portion of the lead frame shown in FIG. 10, i.e., only the peripheral portion of the stage 10 and the tip end portions of the leads 10a on the stage 10 side.
In this case, as shown in FIG. 11, a column portion 120 whose top end face comes in touch with a surface of the stage 10 is formed on the mask member 102 such that the plated metal is not deposited on the surface of the stage 10 other than its peripheral surface. When the lead frame is sandwiched between such mask member 102 and the pressing plate 100, an annular space 110 surrounding the column portion 120 is formed by the recess portion 108 formed in the mask member 102.
The electrolytic plating solution injected from the injection nozzle 112, which is provided at a right angle to the lead frame, is supplied into the annular space 110 shown in FIG. 11 from the supply opening 104. The electrolytic plating solution supplied into the space 110 is discharged from the discharge openings 106 while depositing the plated metal on the surface of the peripheral portion of the stage and the surfaces of the tip end portions of respective leads 10a on the stage 10 side.
However, it is found that, according to the selective plating apparatus shown in FIG. 11, unevenness in thickness between the plated metal layer formed on the stage 10 and the plated metal layers formed on respective tip end portions of the leads 10a is further increased.